Conventional optical fiber communications systems employ optical fibers to transport information in optical telecommunication networks. An electrical signal carrying information is used to modulate the light emitted by an optical source, typically a laser diode. The modulated light is then propagated through an optical fiber link comprising, in modern systems, at least one erbium-doped fiber amplifier (EDFA) and, in some systems, dispersion compensation modules (DCM). The light emerging from the optical fiber link illuminates an optical detector converting the information encoded on the optical signal back into an electrical signal. In early development stages of optical communication systems, the only way to increase the bit rate was to increase the modulation speed of the laser. In the later evolution of multichannel fiber transmission systems, two distinct methods of multiplexing data have been introduced: wavelength division multiplexing (WDM) and coherence division multiplexing (CDM).
Currently, WDM communication systems are the only multichannel optical systems deployed commercially. To increase the optical fiber capacity, WDM communication systems employ multiple lasers and wavelength-selective passive components to multiplex and demultiplex a plurality of distinct optical channels onto a single fiber. A plurality of laser sources, each modulated by a single information channel, have distinct frequencies lying on an internationally agreed frequency grid, and are typically separated by 50, 100 or 200 GHz within the transparency range of the optical fiber.
A traditional WDM communication system comprises a plurality of WDM transmitters, a wavelength division multiplexer and a wavelength division demultiplexer interconnected by an optical link, and a plurality of WDM optical receivers.
Each WDM transmitter operating at a specified distinct wavelength is capable of accepting an electrical input carrying an information channel. If the information channel is coded in an optical domain, then the optical signals have to be converted into an electrical domain by plurality of transponders to drive WDM transmitters. The number of individual information channels in modern WDM communication systems varies from 8 to 128.
A conventional optical link comprises one or more spans. Each span customarily comprises at least one optical amplifier (EDFA), a segment of optical fiber, and, optionally, a dispersion compensation module (DCM). The number of spans depends on the WDM system design and length of the transmission line. For a conventional long haul link, each span has a length of between 80 and 120 km. The maximum length of a link, which is determined by the requirement to regenerate the optical signal, is typically about 600 km. The multiplexed optical signal transmitted via the optical link is routed to the wavelength division demultiplexer for demultiplexing back into individual channels. In each individual channel, the optical signal is received and detected by a respective WDM receiver. A number of WDM receivers corresponds to the number of WDM transmitters. Each WDM optical receiver detects the respective optical signal and processes it to provide recovered clock and data for the subsequent system electronics.
WDM communication systems though significantly enhancing capacity of communication networks have certain technical limitations. To add more WDM channels to the system, one has to broaden the optical bandwidth determined by the spectral band of the optical amplifier, or reduce spacing between the adjacent channels. Broadening the spectral band of the optical amplifier requires new types of amplifiers operating in a wider band than conventional EDFAs. To reduce spacing between WDM channels, new WDM transmitters, multiplexers and demultiplexers should be used with narrower transmission band and tight performance specifications which are not commercially available at this time. In multichannel WDM systems, a substantial inventory of spare parts is required with specific optical characteristics, such as WDM transmitter wavelength. Beyond these technological difficulties, there are principal limitations, such as nonlinear effects and optical dispersion. Nonlinear effects in the optical link, particularly four-wave mixing, cause channel cross-talk and lead to significant performance degradation for the overall system. The effect of four-wave mixing is intensified as the number of equally spaced (in frequency) channels increases and (or) as the power per channel increases. Non-zero fiber dispersion is vital for minimization of nonlinear effects. In some types of the optical fiber, such as Dispersion Shifted Fiber (DSF) having low dispersion in the range of 1550 nm, multichannel WDM transmission is not feasible.
Another approach to extend the WDM system capacity is to introduce additional coding in time domain [T. W. Mossberg et. al., “Lightwave CDMA as a New Enabler of Optimal WDM System Design”, Proc. NFOEC, v.2, p. 369, 1999]. This method allows to insert several information channels into one WDM channel but requires propagation of laser pulses much shorter than the inverse bit rate through the optical fiber.
The CDM approach to capacity enhancement is based on phase modulation of partially coherent light. The CDM method allows to encode, multiplex, transmit and decode several information channels sharing the same optical bandwidth. In CDM communication systems, the encoding and decoding is typically performed using path-length-mismatched Mach-Zehnder interferometers. The number of channels, each having a unique time delay and sharing the same optical bandwidth, is limited by channel bit rate and tolerated cross-talk between the channels.
The CDM systems having certain advantages compared to the WDM systems also have limitations that undermine their practical implementation. The CDM approach does not require strict control of the source optical frequency, as in WDM. However, the CDM communication system performance is defined by spectral shape of the source, path length mismatch accuracy, and the alleviation of environmentally induced phase deviations. In CDM systems, optical signal-to-noise ratio is degraded due to large noise introduced by mutually incoherent optical sources. The performance of CDM systems suffer from significant power budget penalties due to splitting and recombining of the optical power [R. H. Wentworth, “Optical Noise in Interferometric Systems Containing Strongly Unbalanced Paths”, Ph.D. Thesis, Stanford University, 1988], and references therein.